A thermal tool for direct investigation of the interior of the earth

Summary Direct access to the crust and the upper portion of the mantle may be achieved by letting a high temperature (>1100°C) reactor core melt the rock in which it is placed and fall through the resulting magma. Data gathering and retrieval seem feasible. A schematic design of the proposed instrument is given.There are many problems concerning the composition and conditions of the interior of the earth which will not be solved upon completion of the projectedMohole Project. Comparison of the continental crust with the oceanic crust, relative distribution of radioactivity under continents and oceans, and the investigation of the mantle itself require access to greater depths than the present drilling techniques permit. To achieve these aims, it is recommended that a dense, heatgenerating object (such as a nuclear reactor core) be placed in the top of a salt dome. The hot object would melt the salt and fall downward through the moten salt. The sinking object would pass out of the source salt bed into rock at such a depth, say 35 000 feet, that if a few percent of H₂O is present at that depth, then a granitic rock would melt at about 700°C. However, encounter with SiO₂ containing no water would require a much higher temperature of about 1700°C. The type of rock that actually exists immediately below the source salt bed is unknown, but it is probably not a granitic rock.Thermal considerations indicate that the hole will freeze shut after downward passage of the tool, leaving the tool inside a liquid bubble. If the tool can generate heat long enough to melt its way up, as well as down, it may be possible to obtain magma samples. Instrumentation for control and telemetry purposes appears extremely difficult. Initial emphasis is placed on attaining the depth of interest.